In January 1912, Violet Asquith and Venetia Stanley, best friends and secret lesbian lovers, visit Sicily with Violet's father, British Prime Minister H. H. Asquith, and Edwin Montagu (a young Liberal MP and protégé of Asquith). Both Asquith and Edwin fall in love with Venetia. After they return to the UK, Asquith grooms Venetia towards a sexual relationship. Venetia is simultaneously flattered (she is pleased to have the most powerful man in the world infatuated with her) and repelled (Asquith is 35 years Venetia's senior). By July 1912, Asquith has begun an intimate affair with Venetia.
Venetia finds herself caught between Asquith, Edwin, and Violet. Edwin proposes marriage to Venetia: she declines since she does not love Edwin and because Edwin will lose his inheritance if he marries a Gentile. However, Venetia remains friends with Edwin, who still hopes he will eventually marry Venetia.
When war breaks out in August 1914, Asquith's letters to Venetia—already long and frequent—arrive at least daily. Besides proclamations of love, Asquith reveals state secrets to Venetia and asks for her political advice. Overwhelmed and frightened by Asquith's neediness, Venetia attempts to put a brake on her relationship with Asquith by training as a VAD nurse. When that does not deter Asquith, Venetia accepts Edwin's marriage proposal, even though she would have to convert to Judaism. However, Venetia holds off from telling Asquith about her engagement.
On the evening of May 4, 1915, Asquith discovers Venetia and Violet having sex. (This is the point of divergence from our historical timeline.) Asquith is horrified: he realizes too late that he seduced Venetia because he had suppressed his incestuous lust for Violet. In retaliation for Asquith's tantrum when he discovered Venetia was Violet's lover and Edwin's fiancée, Venetia impulsively gives to Lord Northcliffe, the anti-Asquith publisher of the Daily Mail, the letters Asquith wrote to her. Although Venetia almost immediately regrets it, Northcliffe publishes extensive extracts from Asquith's letters to Venetia. This sparks a scandal and Asquith is under pressure to resign because he violated the Official Secrets Act 1911. Instead, Asquith shoots himself.
Venetia's father, Lord Sheffield, forces Venetia, who is now lambasted as a femme fatale, to exile herself on the Stanley family estate on the Welsh island of Anglesey. Edwin jilts Venetia. Venetia becomes a philanthropist for the people of Anglesey. In 1939, Cicely Asquith, a (fictional) granddaughter of the late prime minister and Somerville student who is writing a history thesis on her grandfather's downfall, contacts Venetia. To avenge herself on the Asquith family, Venetia seduces Cicely, but soon Venetia becomes genuinely fond of her. Cicely, who becomes a diplomat and novelist, is torn between her deep Catholic faith (Cicely's mother had converted to Catholicism and raised her children in that church) and her lesbianism. In May 1947, Cicely throws herself from the Empire State Building. Shortly afterwards, Venetia discovers she is dying of breast cancer. Violet, who has become a prominent Liberal MP, reconciles with Venetia shortly before Venetia's death in August 1948.